Breast cancer causes
It is still not clear why some women and men develop breast cancer, but a number of risk factors have been identified.
Women who are told they are at a higher risk of developing breast cancer may never develop it, while others with a lower risk do.
Women will be invited for breast screening from their 50s to check for early signs of breast cancer - invitations will be offered to women from age 47 in the near future. However, women thought to be at a higher risk of developing breast cancer may be screened earlier.
Causes of breast cancer
Among the most significant factors are advancing age and a family history of breast cancer. Risk increases slightly for a woman who has had a benign breast lump and increases significantly for a woman who has previously had cancer of the breast or the ovaries.
A woman whose mother, sister, or daughter has had breast cancer is two to three times more likely to develop the disease, particularly if more than one first-degree relative has been affected. Researchers have now identified a number of genes increase the risk of familial breast cancer, including BRCA1 and BRCA2. Having a high-risk gene predisposes a woman to breast cancer but does not ensure that she will get it. These genes cause about 1 in 20 breast cancers.
Generally, women over 50 are more likely to get breast cancer than younger women, and black women are more likely than white women to get breast cancer before menopause.
A link between breast cancer and hormones is gradually becoming clearer. Researchers think that the greater a woman's exposure to the hormone oestrogen, the more susceptible she is to breast cancer. Oestrogen tells cells to divide - the more the cells divide, the more likely they are to be abnormal in some way, possibly becoming cancerous.
A woman's exposure to oestrogen and progesterone rises and falls during her lifetime, influenced by the age she starts and stops menstruating, the average length of her menstrual cycle, and her age at first childbirth. A woman's risk of breast cancer is increased if she starts menstruating before the age of 11, has her first child after 30, stops menstruating after 55, or has a menstrual cycle shorter or longer than the average 26-29 days. There seems to be a small increase in risk while a woman is taking the combined oral contraceptive pill - but the increase in risk goes back to normal 10 years after stopping taking it. Balanced against this, the pill also seems to reduce the risk of some other cancers, such as ovarian cancer. Some studies suggest that taking hormone replacement therapy (HRT) after menopause may increase risk, especially when taken for more than five years. Heavy doses of radiotherapy may also be a factor, but low-dose mammograms pose almost no risk.
Obesity increases the risk of postmenopausal breast cancer.
According to Cancer Research UK drinking alcohol increases the risk of breast cancer with the risk increasing as the amount of alcohol consumed increases. Smoking also increases the risk of breast cancer.